The present invention is in the field of regulating the average power applied to varying loads; particularly, this invention is applicable to oil-field treater equipment.
An example of the type of equipment in which the present invention finds it primary, though not exclusive, application is Prestridge, U.S. Pat. No. 3,772,180. The Prestridge apparatus is an oil treater that reduces the amount of water contained in oil. It applies an AC voltage across an interface in the vicinity of an oil-water emulsion. The amount of current that flows through the emulsion varies widely according to how much moisture is in the emulsion. As a result, the amount of power drawn by this load can swing to rather high levels when the moisture content of the emulsion is high. If the average power applied to the load remains high, damage can be done to the treater equipment. Accordingly, it is necessary to regulate the power applied to the load.
Another patent issued to Prestridge, U.S. Pat. No. 3,939,395, discloses a circuit for regulating the average power applied to the load. Basically, the apparatus in the U.S. the No. 3,939,395 patent senses te AC current flowing through the load, rectifies and filters the representation produced by the sensing, and uses the result as an indication of whether or not to remove the power from the load. As long as this resultant filtered signal remains above a predetermined level, the power remains removed from the load. When the filter signal falls below the predetermined level, the apparatus reconnects the AC source at a zero crossing of the voltage, thereby avoiding high-voltage transients in the transformer. Because the filtered signal is at a high value immediately after a large current has flowed through the load, the power is disconnected for some period of time until the voltage in the filter circuit has decayed below the predetermined level. If the current is relatively high, it takes a relatively long time for the voltage in the filter circuit to decay, thereby decreasing the duty cycle of the application of power to the load. If the current level is not quite so high, the duty cycle would be higher. If the current is low enough to allow continuous application of the source voltage without exceeding the average power, the voltage in the filter circuit would not be high enough to cause the power to be disconnected from the load, and the duty cycle is 100 percent. Thus, the average power level remains below that which would damage the treater apparatus.
While the apparatus of the U.S. Pat. No. 3,939,395 patent does act to protect the treater, there are two areas in which improvement can be effected. The first improvement involves the average power resulting from the regulating mechanism. Although the apparatus of the U.S. Pat. No. 3,939,395 patent keeps the average power below the danger level, different moisture levels in the emulsion result in different average powers when the connecting and disconnecting mechanisms are brought into play. Accordingly, there are ranges of operation in which the amount of power applied is decidedly less than the highest permissible level. Accordingly, a less-than-optimum level of treater action results. The second area open to improvement is in the maintenance of the apparatus; the SCR's used in the apparatus have a higher rate of malfunction than is desirable.